This invention relates to the preparation of N-alkyl hydroxylamines by the catalytic hydrogenation of a nitroalkane.
Hydroxylamine and its N-alkyl derivatives are valuable oxygen scavengers in boiler feed waters (see, for example, U.S. Pat. Nos. 4,067,690 and 4,350,606) with the N-alkyl hydroxylamines being particularly useful. They are conventionally added to the boiler waters as an aqueous solution of the free base, or, optionally (but less preferably) as the salt. The free base form is regarded as greatly preferred for this use as salt residues are undesirable in the boiler. The salts must be converted to the free base before use, thus adding another process step either at the processing site or at the application site.
Various methods for synthesizing alkyl hydroxylamines are known. Catalytic hydrogenation of a nitroalkane to the corresponding N-alkylhydroxylamine has received considerable attention. This process, as presently conducted, has several serious disadvantages, including: (1) the yield is relatively low; (2) substantial amounts of alkylamine are formed; (3) the free base, when finally recovered, is unstable (that is, not stable under ordinary storage conditions which such material must undergo); and (4) the catalyst rapidly loses its activity and selectivity.
A method of stabilizing the desired N-alkyl hydroxylamines was described in U.S. Pat. No. 3,173,953, to J. R. McWhorter. McWhorter requires the hydrogenation of nitromethane to be conducted with Pd/C in the presence of sulfuric acid to form N-methylhydroxylamine sulfate salt. The sulfuric acid reacts with the N-methylhydroxylamine as it is formed to form the corresponding salt and thereby takes it out of the reaction and prevents overreduction to methylamine. A chelant may be present to remove impurities contributed by the water and the apparatus. The reference shows, by example, the ability to attain high yields of N-methylhydroxylamine sulfate salt (Example 1) but additional steps, including neutralization with a base, are of course necessary if free N-methylhydroxylamine base is to be recovered. These additional steps reduce the overall yield of the free base and may introduce stability-jeopardizing impurities.
Koperska et al., in Przemysl Chemiczny, 49(10), 594-598 (1970), report that hydrogenation of nitromethane over Pd/BaSO.sub.4 yields methylhydroxylamine. Although the synthesis was performed both with and without the use of acid, there was no provision made to eliminate the presence of transition metal ions during the formation of the hydroxylamine. In addition, the authors stated that the Pd/BaSO.sub.4 catalyst could not be re-used. They also disclosed the use of various other hydrogenation catalyst systems with varying results. For example, Pb/C gave low yield but could be re-used if fresh make-up catalyst was added. One of their poorest performing catalyst was Pd/Al.sub.2 O.sub.3 which gave a yield of methylhydroxylamine of about 15% after 7 hours. Yield of byproduct amine in all cases was at least about 10%.
Johnson et al., J. Chem. Soc., pp. 1093-1103 (1956) indicated that the presence of metal catalyst positively influences the oxygen uptake of hydroxylamines.
The instability of aqueous solutions of N,N-dialkylhydroxylamines also been long recognized. U.S. Pat. No. 3,333,001 (1967) proposed to protect such aqueous solutions against storage instability by the addition of a small amount of a benzothiazole, e.g., mercaptobenzothiazole.